Income Tax - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Taxes Disclosure [Line Items]
Income tax expense	$ 249	$ 259	$ 548	$ 559
Effective tax rate	37.00%	37.00%	35.00%	36.00%
Requested refund adjustment from settlement with taxing authority	$ 30		$ 30
Estimated liability for uncertain tax positions	34		34
Uncertain tax positions, reasonably possible to resolve within twelve months	25		25
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	16		16
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate	34		34
Total amount of unrecognized tax benefits, if recognized, that would reduce effective tax rate, net of federal taxes	22		22
CECONY
Income Taxes Disclosure [Line Items]
Income tax expense	$ 222	$ 233	$ 515	$ 496
Effective tax rate	37.00%	37.00%	35.00%	35.00%
Estimated liability for uncertain tax positions	$ 2		$ 2
Uncertain tax positions, reasonably possible to resolve within twelve months	2		2
Uncertain tax positions, reasonably possible to resolve within twelve months, net of federal taxes	$ 1		$ 1